1. Field of the Invention
The present invention relates to a process for the continuous preparation of dialkyl carbonates by reaction of the corresponding alkanol with oxygen and carbon monoxide in the presence of a copper-containing catalyst suspended or dissolved in the reaction medium, at elevated pressure and elevated temperature, in which the catalyst can be eliminated from the reaction medium by sedimentation and the reaction water in the particularly important case of methanol can be eliminated from the reaction medium by simple distillation.
2. Description of the Related Art
In the past years a series of processes have been developed for the preparation of dialkyl carbonates by the catalytic reaction of the starting materials alkanol, carbon monoxide and oxygen.
In DE-A 2 110 194, suitable catalysts mentioned are metal complexes selected from groups IB, IIB and VIIIB of the Periodic Table of the Elements, in particular those of the metals Cu, Ag, Au, Zn, Cd, Hg, Fe, Co and Ni, which can exist in two different oxidation states in redox reactions.
This process gives good yields with Cu.sub.2 Cl.sub.2, but has the disadvantage that the separation of the highly expensive complex ligands and of the dissolved complexed catalyst from the reaction solution is laborious.
In DE-C 2 743 690, instead of the copper complex compounds, simple monovalent salts of copper are used as catalysts. Although this process variant gives good yields of dialkyl carbonates, the work-up of the reaction solution also poses great problems here, since the partially dissolved catalyst must be separated from the reaction solution. According to the teaching of the patent, this is carried out by filtering off the suspended fraction and rectifying or crystallising the dissolved catalyst. Since the catalyst-containing reaction solutions carry the catalyst into further parts of the plant, a high expenditure in terms of apparatus is required for the work-up of the reaction solution and of the catalyst. Because of the corrosive properties, all apparatuses (tanks, piping, distillation, crystallisation and filtration apparatuses) which come into contact with the catalyst must be composed of corrosion-resistant material. As a result, the process loses its attraction.
The same work-up problems also cause the use of synthesis gas instead of CO, as described in DE-C 3 045 767, to remain economically unattractive. The corrosion problems caused by the copper-containing catalyst during the workup of the reaction solution cause processes which include further additions to the catalyst (for example EP-A 217 651, EP-B 90 977, US-A 4 370 275) to be uneconomic.
DE-A 3 926 709 teaches a processing alternative to separating off the catalyst; in this case the copper-containing catalyst remains in the reactor. The dialkyl carbonate formed during the reaction, together with the reaction water and alkanol, is stripped from the reaction mixture by the reaction gas. This effect is generally achieved in that a gas stream of 20 to 30 l(S.T.P.)CO/O.sub.2 of gas mixture per g of copper present in the reactor as copper catalyst is passed through the reaction mixture. Disadvantages of this process are the very high gas quantities which must be maintained in the circulation and the high energy costs caused as a result, and the problems of gas dispersion caused by the high gas quantities. In this procedure, the temperature and pressure of the reactor must additionally be controlled very exactly in order to be able to maintain the liquid level in the reactor, since even small variations in reactor temperature or in pressure lead to markedly changed output rates. Moreover, in this case, a relatively large quantity of water accumulates in the reactor, which decreases the selectivity of the reaction.
The reaction is carried out in the described processes in autoclaves as pressure vessel. These expensive reactors have the disadvantage that the mixing of the reaction medium is carried out with the aid of stirrers and internals which are subjected to high wear. In addition, the stirrer unit especially represents a potential source of leakage.
EP 460 732 Al and EP 460 735 A2 describe a process in which, analogously to DE-A 3 926 709, the dialkyl carbonate formed during the reaction, together with alkanol and reaction water, is eliminated from the reactor by the reaction gas. EP 460 735 A2 describes a specially dimensioned loop reactor having an external material circulation, which is said not to have the abovementioned disadvantages. However, this reactor easily forms dead zones, especially in the lower bend region, has a high volume, an additional heat exchanger and leakage problems because of the many flange joints. The process described in the two patent applications operates with very high water contents in the reactor, which, according to U. Romano, R. Tesel, M. M. Mauri and P. Rebora (Ind. Eng. Chem. Prod. Res. Dev., (1980), 19, 400), leads to a loss in CO selectivity and increased CO.sub.2 production. In EP-A 460 735, for example, in Example 1 a CO selectivity for DMC of only 77% is achieved; the remaining selectivity is apportioned to CO.sub.2 formation. In EP-A 460 732, the DMC selectivities achieved with respect to methanol are only 95-97%, as follows from the examples. These disadvantages and the disadvantages already listed for DE-A 3 926 709 mean that this process variant is uneconomic.
A further process problem is separating off the reaction water, for example from methanol and dimethyl carbonate. According to German Offenlegungsschrift 2 450 856, separating off the dimethyl carbonate from the reaction water and methanol using a simple rectification becomes complicated, since various azeotropes are formed between DMC, MeOH and water. The patent application teaches separation by use of an extractive distillation using water as solvent. However, the process is uneconomic, since considerable water quantities are required for the separation (9.5 g of water for 1 g of reaction solution).